1. Field of the Invention
The present invention relates to optical systems, and more particularly, to an optical system and a projector.
2. Description of the Prior Art
A projector takes a video source and projects corresponding images onto a projection screen using a projection lens system. In some cases, the projector placing may deviate from the centerline of the projection screen. Referring to FIG. 1 as an example, the projector 100 is placed on a plane 110 and some of the projection light beam is blocked by the plane 110 due to the improper placement of the projector 100. Therefore, the projector 100 needs to be placed at a tilted angle so that the placement deviates from the centerline of a projection screen 120. This is called “tilted projection”, which will result in a distorted trapezoidal shape projected image on the projection screen 120. This kind of distortion is the well-known “keystone distortion.”
To ameliorate the keystone distortion, there are two conventional correction methods: digital and optical. Digital keystone correction method involves a scaling/compression algorithm being applied to the image before it is projected on the screen. In effect, the compression algorithm squeezes the entire image down to the thinnest edge of the trapezoidal projected image. While the approach achieves the desired rectangular projected image, it does have some undesirable side-effects, including lowering the resolution and thus degrading the quality of the projected image on the projection screen.
The optical keystone correction method is more commonly applied in LCD projectors. The optical keystone correction method refers to the horizontal or vertical movement of the projection lens with its housing. The optical keystone correction method provides better keystone correction effect without reducing resolution; however, this requires the projection lens to be designed with a larger field, and leads to a projection lens with a larger volume.
Evidently, there are still some problems in the conventional keystone correction methods that need to be overcome.